In Sanford, Florida, jury selection begins today in the murder trial of George Zimmerman. The neighborhood watch volunteer is charged with the shooting death of a 17-year-old, Trayvon Martin, last year. Police, at first, declined to charge Zimmerman because of Florida's Stand-Your-Ground law. It gives immunity to people who use deadly force in self defense.

After weeks of petitions and protests, Florida's governor gave the case to a special prosecutor who did charge Zimmerman with second degree murder. Despite all the media attention and public outrage over how the case has been handled, NPR's Greg Allen reports that Zimmerman's guilt or innocence rests on a fairly narrow set of facts.

GREG ALLEN, BYLINE: It's a case that began in February of last year, when George Zimmerman called police to report someone in his neighborhood he thought looked suspicious.

GEORGE ZIMMERMAN: This guy looks like he's up to no good or he's on drugs or something. It's raining and he's just walking around looking about.

ALLEN: Several minutes later, Zimmerman and Trayvon Martin, an African-American teenager, began fighting. The fight ended with Martin dead from a single gunshot. Zimmerman's lawyer, Mark O'Mara says he believes this is a straightforward case of self defense.

MARK O'MARA: Trayvon Martin was not profiled because he was black. George is not a racist. It was an event where two lives intersected and one of them got extraordinarily aggressive. The evidence supports that was Trayvon Martin.

ALLEN: In the fight, George Zimmerman sustained injuries to his face and the back of his head. Police photos and a medical report back that up. And there's something else. There were only two people who witnessed his confrontation with Trayvon Martin from start to finish and the other person is dead. Brian Tannebaum, past president of the Florida Criminal Defense Lawyers Association, says he believes this makes it a tough case for prosecutors.

BRIAN TANNEBAUM: I don't know how they're going to refute a lot of what George Zimmerman is saying, because there's no video, there's no other evidence. I know there's a cell phone call and someone on the phone and who was talking and this and that. But here's the guy who was there.

ALLEN: Zimmerman's defense team opted not to ask a judge to dismiss the charges against their client in a separate Stand-Your-Ground hearing. O'Mara says they want the case decided, not by a judge, but by a jury. But included in the evidence the jury will likely consider is this exchange between Zimmerman and a police dispatcher.

UNIDENTIFIED MAN: Are you following him?

ZIMMERMAN: Yeah.

UNIDENTIFIED MAN: OK. We don't need you to do that.

ALLEN: Supporters of Martin's family say this is evidence that Zimmerman helped precipitate the confrontation. Even if that's so, under Florida Stand-Your-Ground law, Zimmerman still has a right to defend himself. In recent weeks, as the trial approached, O'Mara has sought to introduce text messages, school records and other evidence that showed Trayvon Martin used marijuana and had some history of fighting.

That lead Benjamin Crump, a lawyer representing Trayvon Martin's family, to charge Zimmerman's defense is trying to distract from the facts of the case.

BENJAMIN CRUMP: Trayvon Martin did not have a gun. Trayvon Martin did not get out the car to chase anybody. Trayvon Martin did not shoot and kill anybody. Trayvon Martin is not on trial.

ALLEN: Judge Debra Nelson has ruled information about Martin's fighting and marijuana use won't be allowed in opening arguments, and later, only if prosecutors, in her words, open the door in some way. Tannebaum says that's a risk prosecutors will have to guard against.

TANNEBAUM: If I start to portray Trayvon Martin as an angel, as a great kid, then you open the door to the defense coming in and saying, well, wait a second. We have some information that he's not who you portray him.

ALLEN: Another difficulty for prosecutors, has to do with one of their key witnesses, a young woman who was on the phone with Trayvon Martin when he and George Zimmerman began fighting. She says Zimmerman chased Martin and may have started the fight. But her credibility is in question since she's admitted to lying to prosecutors about other parts of her story.

In the murder trial expected to last four to six weeks, one of the most compelling pieces of evidence, though, is a 911 call to police just seconds before Martin's death.

(SOUNDBITE OF 911 CALL)

UNIDENTIFIED WOMAN: So you think he's yelling help?

UNIDENTIFIED WOMAN #2: Yes.

UNIDENTIFIED WOMAN: All right. What is your...

ALLEN: In that 911 call made by a neighbor, someone is heard repeatedly calling for help. The question is, whose voice is heard on the recording? The defense says audio tests are inconclusive and have challenged the testimony of prosecution experts who say they believe the voice belonged to Trayvon Martin. Judge Nelson still has to rule on which, if any of the experts, jurors will be allowed to hear.

But both Zimmerman's and Martin's family members are expected to testify. How jurors react to their testimony and the 911 tape, the last seconds of Trayvon Martins life, may help determine the outcome of the case. Greg Allen, NPR News, Miami. Transcript provided by NPR, Copyright NPR.